1. Field of the Invention
The present invention concerns a gear shift control system for an automatic transmission and, more specifically, concerns an improved automatic transmission gear shift control system for alleviating shift shocks of an automatic transmission.
2. Description of Related Art
In a typical automatic transmission, a gear shift is performed to unlock a frictional element which places the transmission into one of the available gears and selectively lock a frictional element which places the transmission into another one of the available gears in order to form a power or torque transmission path for the gear. In such a gear shift process, it is necessary to properly synchronize the locking of selected frictional elements for providing a selected gear with the unlocking of the frictional elements locked prior to the gear shift. In a case in which the locking and unlocking of the selected frictional elements is not properly synchronized such as, for instance, when unlocking of the selected frictional element is too fast, the torque transmission path transitionally passes through a neutral state during the gear shift. This results in an occurrence of what is called a "shooting-up of rotation" in the automatic transmission during the gear shift. The term "shooting-up of rotation" as used herein shall mean and refer to a rapid increase in the frequency of rotation. On the other hand, if locking of the selected frictional element is invoked before unlocking of the previously locked frictional element has properly progressed, the torque transmission path is locked in accordance with the locked condition of these frictional elements, so that what is called a "slump of rotation" occurs in the automatic transmission during the gear shift. The term "slump of rotation" as used herein shall mean and refer to a rapid decrease or slump in the frequency of rotation. As a result of these rapid changes in rotation, i.e., shooting-up and slump of the rotation, shift shocks are experienced.
A mechanism or system for effectively alleviating or eliminating shift shocks has been proposed in Japanese Unexamined Patent Publication No. 63-214550. In this system, the locking speed of frictional elements during a gear shift is controlled in such a way that the locking is initially speeded up or set to a high level of speed over a specified period of time (hereafter referred to as a "specified speed-up time period") and is subsequently slowed down or set to a low level of speed in order to produce a quick locking of the frictional elements in the first half of a locking phase and a slow locking of the frictional elements in the second half of the locking phase, respectively. Simultaneously, the rotational speed input to the transmission gear mechanism is detected in such a way that the specified speed-up time period over which the locking of frictional elements is speeded up is sequentially calibrated in accordance with changes of the input rotational speed. In other words, when a shooting-up of the input rotation is observed in the automatic transmission, the specified speed-up time period is prolonged so as to accelerate locking of frictional elements. On the other hand, when a slump of the input rotation is observed in the automatic transmission, the specified speed-up time period is shortened so as to decelerate the locking of the frictional elements. By repeating the prolongation and shortening of the specified speed-up time period, shooting-up or slumping of the input rotation is suppressed or prevented and shift shocks are effectively reduced or eliminated.
When the specified speed-up time period over which the locking of frictional elements is accelerated is sequentially calibrated in accordance with changes in the input speed of rotation, a gear shift, for which a sequential calibration of the specified speed-up time period must be conducted, is present. For instance, a third-to-second gear shift-down may include not only an ordinary third-to-second gear shift-down (which is hereafter referred to as an ordinary 3-2 shift-down) in which the automatic transmission changes or shifts from a normal state, in which the third gear has been established, to the second gear, but also a continuous gear shift-down from the fourth gear to the second gear via the third gear (which is hereafter referred to as a continuous 4-3-2 shift-down) which takes place over a relatively short time period. In a continuous 4-3-2 shift-down, the line pressure of hydraulic control circuit drops as a result of supplying of a large quantity of a hydraulic working oil into the frictional elements which form the third gear during the continuous 4-3-2 shift-down. Accordingly, if the continuous 4-3-2 shift-down occurs gradually over an extended time period, the application of hydraulic working oil to the frictional elements which form the third gear is completed before the beginning of the 3-2 shift-down, so that the line pressure is elevated and restored to the normal level by a pressure regulator valve of the hydraulic control circuit. Consequently, the gear shift occurs under the normal level of line pressure. However, if the continuous 4-3-2 shift-down is made quickly in a short time period, the second gear is provided while the line pressure is restored to its normal level, i.e., under a lower level of line pressure. If the specified speed-up time period over which the locking is speeded up is sequentially calibrated even when the continuous 4-3-2 shift-down occurs over a short time period in the same manner as in an ordinary 3-2 shift-down, information about a gear shift performed with a different line pressure is obtained and, as a result, it may become impossible to sequentially calibrate the specified speed-up time period properly.